This paper presents a new trust evaluation scheme in an ad hoc network To overcome the limited information about unfamiliar nodes and to reduce the required memory space, we propose a cluster-based trust evaluation scheme, in which neighboring nodes form a cluster and select one node as a cluster head The head issues a trust value certificate that can be referred to by its non-neighbor nodes. In this way, an evaluation of an unfamiliar nodes trust can be done very efficiently and precisely In this paper we present a trust evaluation metric using this scheme and some operations for forming and managing a cluster An analysis of the proposed scheme over some security problems is also presented.